Manufacturers of integrated circuits desire lithographic techniques which achieve high resolution printed patterns. One conventional lithographic technique is photolithography. However, printed patterns produced by current conventional photolithographic techniques achieve sub-micron resolution only with great difficulty because the natural phenomenon of diffraction causes a poor image quality and a small depth of focus.
Other technologies have been developed which improve upon the resolution achievable by photolithographic techniques. For example, X-ray lithography can achieve sub-micron resolutions. However, X-ray lithography is particularly expensive and suffers from problems related to generating X-rays with sufficient brightness to effectively expose a resist.
Electron beam lithography, when used in a serial writing mode, also improves upon the resolution attainable with photolithographic techniques. However, this technique is too slow for practical use in volume production. While parallel-printing with electron beam lithography is capable of high resolution, it has critical dimension control problems due to a proximity effect inherent in the electron beam process. This proximity effect may be corrected with local dose variation in the serial writing mode, but the problem is difficult to control when parallel-printing.
Ion beam lithography is another technique. Ion beam lithography does not suffer from diffraction effects such are as experienced in photolithography. Additionally, ion beam lithography does not suffer from a proximity effect, such as is experienced in parallel electron beam lithography. Moreover, sources for generating highly collimated high energy ion beams are commercially available. Such sources typically cost mush less than the brightest X-ray sources. Moreover, such sources expose a resist more effectively than the brightest of currently available X-ray sources.
However, ion beam lithography suffers from problems related to providing suitable masks for use in parallel-printing operations. Ion beams are easily blocked. Almost any solid material having a thickness of greater than one micron will successfully block the transmission of a high energy ion beam. Accordingly, masks used in ion beam lithography either have voids or extremely thin sections of material in transmissive areas of the mask.
When a mask used in ion beam lithography utilizes thin materials in the transmissive areas of the mask, an undesirable consequence results. Due to nuclear scattering effects, this type of mask tends to cause a highly collimated ion beam to diverge. In general, masks are spaced apart from a surface upon which a pattern is being printed. The spacing prevents damage from occurring to the surface or to the mask. Due to this spacing and the divergence caused by such a mask, highly precise printed features are not obtainable.
The use of voids in transmissive areas of a mask is therefore desirable because a highly collimated ion beam passing through a void in a mask remains highly collimated. However, many patterns are impossible to achieve using simple voids for the transmissive areas of the mask. For example, a doughnut-shaped transmissive area would leave no mechanical support for a nontransmissive "doughnut hole". Consequently, such a shape cannot be printed using a simple void for the transmissive area. Moreover, many structures which are physically possible to construct using void transmissive areas are too dimensionally unstable to be of practical value. In other words, some solid portions of the mask cannot be forced to maintain a precise position relative to other solid portions of the mask.
To solve the problems of void transmissive areas in ion beam masks, two conventional solutions are known. One solution utilizes multiple, complementary, stencil-type patterns which are used in multiple, sequential, registration and exposure steps to achieve a single printed pattern. Another solution utilizes a grid, such as a net, screen, or the like for the transmissive areas of the mask. These two solutions tend to suffer serious drawbacks when used in ion beam lithography. In particular, the multiple, complementary stencil-type mask technique suffers from excessive complication caused by the multiple exposure steps and the multiple registrations of masks. Both solutions suffer from excessive complication in the fabrication of the mask. Moreover, both solutions suffer from a diminished dimensional stability caused both during mask fabrication and as a result of mask heating during the exposure step.
Tensile stress is required in a material from which a mask is fabricated to keep the mask smooth and flat in order to minimize distortion. Stress is relieved wherever voids or grids are formed. The non-uniformity of a printed pattern causes the stress relief to be greater in some areas than others. Consequently, the pattern distorts in an attempt to equalize non-uniformities in stress.
In addition, during the exposure step ion beam radiation which is not transmitted through the mask is absorbed by the mask. Thus, the temperature of the mask increases during the exposure step. The heating of the mask relieves some of the tensile stress of the mask and causes solid portions of the mask to expand. Consequently, nothing prevents the temperature increases from causing the solid portions of the mask to expand in the vicinity of the boundaries between the solid and void areas. This expansion diminishes the precision otherwise achievable using ion beam radiation.
Accordingly, a mask is needed which is dimensionally stable throughout fabrication and use. In addition, such a mask needs to utilize void transmissive areas to minimize ion beam divergence causable by nuclear scattering.